The need to reliably detect bubbles in a moving fluid can be important in many situations. For example, in film coating operations, bubbles existing in the film emulsion at the coating stage can cause streaking in the resultant film coating with consequent loss of film production. Since the detection of bubbles can result in corrective action such as marking of suspected defect areas on film webs for later excission or that otherwise results in temporary interruption of the coating process, it is important that bubble detection operate reliably and not give false readings of bubble detection when, in fact, no bubbles are present in the fluid.
One well known bubble detection technique utilizes doppler frequency shift detection to detect the existence of a moving bubble in the fluid. A transducer is used to transmit and receive bursts of high frequency energy along the length of a conduit in which the fluid is flowing. Reflected echoes received by the transducer are analyzed to find any with characteristic frequency shifts that would indicate the presence of bubbles in the fluid. It will be appreciated that such detection techniques will also detect moving solid particles and reference to bubbles herein should be understood to encompass solid particles as well.
While generally useful for this purpose, it has been found that such systems have detection reliability problems in which there is uncertainty as to whether the detection is indicative of a real event, i.e. a moving bubble or is just a false detection indication due to electrical or mechanically induced noise in the signal.
It is therefore an object of the invention to provide apparatus and methods for detection of bubbles in a moving fluid which provides improved reliability of detection.